Can tickling help us understand some symptoms of schizophrenia?

Schizophrenia is a severe psychiatric disorder that has a variety of symptoms. One of the most commonly occurring symptoms are auditory hallucinations. Scientist believe that impaired self-recognition is a major contributing factor to the experience of auditory hallucinations. Impaired self-recognition is not only a necessary factor for auditory hallucinations to occur; the origin of these events must also be misattributed to something outside of the individual.

Normally, one cannot tickle oneself. A forward model describes why one is able to predict the consequences of one’s own actions. In a forward model, sensory inputs from the somatosensory cortex are compared to the predicted sensory feedback. When the delay between the signals is small enough, this prediction is utilized to attenuate the self-produced sensory signal. Through studies using fMRI, it is believed that this process of comparison takes place in the cerebellum. Other systems that may utilize this same type of model are: attenuations in muscle movements caused by electric shock when the muscle is voluntarily flexed or during head movement when the actual location of an object is identified by comparing the actual image on the retina with a previous instance of the image.

A group of cognitive neuroscientists were interested in whether or not this forward model could explain why patients with schizophrenia experience auditory hallucinations and created an experiment to test this model. A group of 35 patients, each with a clinical diagnosis of schizophrenia, was assigned to different groups based on whether or not the patient was currently experiencing auditory hallucinations (n=20), or had not experienced auditory hallucinations in more than two weeks (n=15). A third group of healthy age-matched subjects were also tested as a control group. Stimulation was applied to each subject’s left palm by either the researcher or the subject. The subject was then asked to rate the type of tactile stimulation perceived. The results are shown in Figure 1.

Figure 1: The mean difference in rating between stimulation produced by the experimenter or the patient. No significant difference in response was given by the patients experiencing auditory hallucinations between the self-produced and externally produced stimulation, thus its difference is near zero.